We have continued our studies on the relationship of suppression of tropomyosin (TM) synthesis to neoplastic transformation. Previous observations have led us to hypothesize that TM suppression is a causal event in neoplastic transformation. We have obtained evidence supporting this hypothesis by restoring expression of TM1, one of two suppressed tropomyosins, in the v-Ki-ras- transformed NIH3T3 cell line, DT, by retroviral mediated cDNA transfer. Cell clones expressing the cDNA had elevated levels of TM1 and lost the ability to grow under anchorage- independent conditions. They also did not participate in formation of tumors in athymic mice. However morphological reversion was incomplete. Elevated levels of TM1 synthesized in the transduced clones were only partially utilized in the cytoskeleton and disrupted microfilament bundles were only partially restored. Abnormal levels of TM1 homodimers and of crosslinked homodimers were produced which may associate poorly with the cytoskeleton. Studies with double insertion of both TM1 and TM2 suggest that under these conditions complete reversion of the transformed phenotype may occur.